In general terms the concept of release is letting something go. So if we release a prisoner we let go of the prisoner we've held it and then we've let it go if we release a piece of rope we've held it on then we've let the rope go. If we release something in a golf swing it's something we've held.

so in the golf swing we hold some angle so during my set up faze I don't have angle during my backswing I create some angle. 90 degrees say between the shaft and the forearm is a good checkpoint for you are enjoying the downswing I maintain that angle to a certain point and then I release it. If I was to take the club to the top and release my angle here it's going to be less efficient than if I release it at the ball and likewise if I have my angle and I release it here it's going to be less efficient than if it was at the ball.